thats so wrong... holy is not only viable but there are fights even preferring holy over disc (iron qon hc, tortos hc).
both specs are fine and viable.

This is where i wish that tri-spec was available. im disc main with a shadow OS, HC iron qon and HC tortos are our next 2 for progression. i play holy pretty well (been disc since wotlk) feeling alot more confident about playing it now i have LMG!

In a 10m setting Disc is far superior to Holy, our main healer in our guild is and my guess is will always be Holy, we recently picked up a disc with way worse gear he can out heal her with ease. THe trouble is from what I have seen, most that play disc do so because they hate disc and will not make the change. I myself have never even played Holy on my priest. He has always been Disc.

I have question, can disc and holy priests work in 10man raid or even 2 both discs priests in 10m ?

Depends on the Boss. We have 4 Healers in our Roster: Shaman, Pala, 2x Disc, and are now 11/13 heroic.
Best Healing setup for most bosses is Hpala+double disc but our Resto is Guild and Raidlead so we play Restom, Hpala, Disc or Resto double disc on most 3 Healer Bosses. We play double disc setup on almost all bosses that dont need a paladin (hello hand of protection).

And yes, disc is far superior to holy in 10m. Healing output is about the same but shields prevents instakills and you deal a lot of damage. With double disc you have >100k dps from your healers in your raid, we managed to kill twin consorts hc with >50 seconds left on the enrage timer and this fight is conisdered a dps check.

Playing holy on horridon is just dumb, smiting on horridon is like throwing lay on hands in the raid.

In a 10 man with a 2 healer set up, if the tank is low and there is no damage taken debuff on the boss, should I be casting flash/gheal at all? Are those spells ever worth it or do I just heal via attonement?

In a 10 man with a 2 healer set up, if the tank is low and there is no damage taken debuff on the boss, should I be casting flash/gheal at all? Are those spells ever worth it or do I just heal via attonement?

If the tank is lower than the rest, atonement is fine, if there's other people nearly as low, but there's only tank damage going on, only then should you use FH/GH. Although honestly I never find myself in that situation, only times I use direct heals is if there's no target for offensive spells, or GH + IF to get a big shield up to avoid damage.

In a 10 man with a 2 healer set up, if the tank is low and there is no damage taken debuff on the boss, should I be casting flash/gheal at all? Are those spells ever worth it or do I just heal via attonement?

if a tank is so low that you can't get him back up with atonement, you're probably going to have to voidshift. that said, your tanks need to be vocal about when their personals are down before a big damage spike so that you can painsup/barrier them. disc is all about being proactive.

In 10-man, yes. Definitely. You should be discipline, no questions asked, no ifs, no ands, no buts. The biggest reason isn't even the mitigation through Spirit Shell, as some fights don't really give you much uptime to SS. The biggest reason is DPS through Atonement healing during low damage situations. Essentially any soft or even hard enrage timer you may come across can become much shorter just by using Smite, Holy Fire, and Penance when Holy Fire's DoT effect is down to keep a tank and raid members topped up while still doing an additional 40-60k DPS.

I love holy. I've played my priest since BC, and holy has almost always been my personal preference. But discipline right now is far too useful in 10-man raiding over holy. The throughput holy provides is equally, or even superiorly, matched by a resto druid, holy paladin, or mistweaver monk. However, the DPS that discipline brings to the table can't be matched by any of those save maybe mistweavers. Essentially, discipline plays now how shadow used to play in BC. They're a support hybrid DPS/healer spec that gives your raid far too much versatility to be passed up.

thats so wrong... holy is not only viable but there are fights even preferring holy over disc (iron qon hc, tortos hc).
both specs are fine and viable.

disc is way, way better on iron qon (especially 10) because it allows you to avoid extra boss abilities with your extra dps. you might do more hps as holy, but it's just healing through mechanics you should have been able to avoid by speccing disc.

tortos is something of an outlier in that it allows any healer to proactively/shield heal. depending on raid comp, having a disc priest atoning the turtles could still be more useful.

so, yeah. one fight it's somewhat better to be holy just because you basically have passive spirit shell regardless of spec, and another fight you're only padding & wasting time if you go holy.

It is going to depend heavily on your raid comp. If your raid comp lacks a burst healer, Holy will be stronger than Disc on any fight where the damage is intense enough that Disc shields won't cover it.

Holy is far from weak or a bad spec. People are simply not playing it cause disc is op cause of +40k dps and top healing with atonement. Then a barrier/spirit shell/ pw:s+body and soul when needed.
So to answer your question, if you are trying to progress, then yes playing holy is not the best option for you.

For example on iron qon, 3 ppl are stacked, you use pw:s on all 3 and they can get an extra stack. You can't do anything close to that as holy. You might have slightly better healing output in the last phase though when it's only the dude and youre all stacked up.

Paragon used double disc + holy paladin on their WF kills of: Council, Megaera, Primordius Dark Animus, Iron Qon and Twins. As did a couple other top10 guilds. And they did that undergeared, but discs scale very hard with gear in 10 man. Disc+holy pally / Double disc+holy pally is basically the strongest healing setup available for 10 man atm (overall, not for every fight). Nobody is saying holy is bad, because it is wrong. A perfectly played holy priest can do amazingly well on 99% of encounters. Its just not as good as disc in a 10 man setting when in the hands of a player who plays both specs equally well.

Edit: But to answer the OP's thread question, if you can play both holy and disc at a high level, then you are doing your 10 man raid a disservice by being holy (from a hard-stare pure statistics VS progression POV) It's as simple as that.

Paragon used double disc + holy paladin on their WF kills of: Council, Megaera, Primordius Dark Animus, Iron Qon and Twins. As did a couple other top10 guilds. And they did that undergeared, but discs scale very hard with gear in 10 man. Disc+holy pally / Double disc+holy pally is basically the strongest healing setup available for 10 man atm (overall, not for every fight). Nobody is saying holy is bad, because it is wrong. A perfectly played holy priest can amazingly well on 99% of encounters. Its just not as good as disc in a 10 man setting when in the hands of a player who plays both specs equally well.

Edit: But to answer the OP's thread question, if you can play both holy and disc at a high level, then you are doing your 10 man raid a disservice by being holy (from a hard-stare pure statistics VS progression POV) It's as simple as that.